45 



cooled slag ; its sudden cooling has also intensified the chemical 

 activity of its constituents so as to give it hy'draulic properties, 

 while part of the sulphur contained in the original slag has 

 been removed. The sole disadvantage of the process of granu- 

 lating slag is that the product contains 20 to 40 per cent, of 

 water, which must be driven off before the granulated slag is 

 sent to the .grinding machinery. 



In practice the granulation of the slaig is effected by directing 

 the stream of molten slag direct from the furnace into a sheet- 

 iron through. A small stream of water flows along this trough, 

 the quantity and rate of flow of the water being regulated so as 

 to give complete granulation of the slag without using an ex- 

 cessive amount of water. The trough may be so directed as to 

 discharge the granulated slag into tanks or into box cars, which 

 are usually perforated at intervals along the sides so as to al- 

 low part of the water to drain off. 



Drying the slag. As above noted, the granulated slag may 

 carry from 20 to; 40 per cent, of water. This is removed by 

 treating the slag in rotary dryers. In practice such driers give 

 an evaporation of 8 to 10 pounds of water per pound of coal. 

 The practice of slag drying is very fully described in Vol. 10 

 of the Mineral Industry, pages 84-95, where figures and de- 

 scriptions of various driers aire also given, with data on their 

 evaporative efficiency. As noted earlier in this article, one of 

 the methods of manufacturing Portland cement from slag puts 

 off the drying of the slag until after it has been mixed with the 

 limestone, and then accomplishes the drying by utilizing waste 

 heat from the kilns. Kiln gases could of course be used anv- 

 way in the slag driers, but it so happens that they have not been 

 so used except in plants following the method in question. 



Grinding the slag. Slag can be crushed with considerable 

 ease to about 50 mesh, but notwithstanding its apparent brittle- 

 ness it is difficult to grind it finer. Until the introduction of the 

 tube mill in fact it was almost impossible to reduce this material 

 to the fineness necessairy for a cement mixture, and the proper 

 grinding of the slag is still an expensive part of the proceiss, as 

 compared with the grinding of limestone, shales, or clay. 



Composition of the limestone. As the slag carries all the si- 

 lica and alumina necessary for the cement mixture, the lime- 

 stone to be added to it should be simply a pure lime carbonate. 



